Midterm 1 Flashcards
How have cities been show to alter the climate?
- increasing temperature
- decreasing relative humidity
- increasing cloudiness and fog
- decreasing solar radiation input
- decreasing overall wind speed
- increasing rainfall
- decreasing snowfall
What are the 5 major ways in which urban areas influence climate?
- Replacing grass, soil and trees with asphalt, concrete and glass
- Replacing rounded trees and bushes with angular buildings
- Introducing artificial heat (cars/buildings)
- Diverting water from infiltrating soul to drains and sewage pipes
- Releasing contaminants into the atmosphere
What is shortwave radiation?
Radiation with wavelengths between 0.15-3.0 micrometers
What is long-wave radiation?
Radiation with wavelengths between 3.0-100 micrometers
What is the Planetary Boundary Layer?
The portion of the atmosphere that is directly impacted by the underlying surface (Example: windspeed + direction, convection)
What is a Black Body?
Hypothetical object that’s surface emits the max amount of energy possible given its surface temperature. Found through Stefan-Boltzman Law
What are Grey Bodies?
The amount of energy emitted by a grey body can be found using the Stefan-Boltzman Law with the added parameter of emissivity
What does the Bowen Ratio express?
The proportion of energy that is used for sensible heat production relative to latent heat production
For Bowen Ratios greater than unity (1)…
…sensible heat is the main transfer and there is a limit to the available water
For Bowen Ratio less than unity (1)…
…latent heat is the main transfer and there is an increased water vapour content
What is the Urban Canopy Layer?
Vertical region extending from the surface to just above the roofs of buildings
What is the Urban Boundary Layer?
Vertical region extending from the Urban Canopy Layer to where the urban area no longer exerts an influence on the atmosphere
What are the conditions to create an Urban Canopy Heat Island?
Large city, clear skies, light winds, just after sunset
What are the components of the Urban Canopy Heat Island?
- Cliff
- Plateau
- Peak
What is the Cliff? (UCHI)
Found at the rural/urban boundary, it is an area with a steep temperature gradient.
What is the Plateau? (UCHI)
Area of study, but weaker horizontal temperature gradient towards the city centre. The uniformity of the plateau depends on urban land use
What is the Peak? (UCHI)
Located at the city core, is the area where the urban maximum temperature is found
How will the temperature change if the air is unsaturated?
The temperature change will be constant at the DALR of 0.98’C/100m
How will the temperature change if the air is saturated?
Will cool at a slower rate, the SALR= 0.65’C/100m (but it varies)
What is the Adiabatic rate?
Temperature changes with volume change
What is the Environmental Lapse Rate?
Approximately 6’C/km or 0.6’C/100m
What creates an unstable atmosphere?
When the ELR is greater than the DALR. This is common during sunny days when there is extreme surface heating
What creates a stable atmosphere?
When the ELR is less than DALR. An extremely stable atmosphere is when the ELR is negative
What creates a neutral atmosphere?
ELR is equal to DALR. Common during cloudy, windy conditions
What can limit the development of vertical temperature variations?
Clouds restrict surface heating and cooling
What are the 5 basic types of pollution plumes?
- Looping
- Coning
- Fanning
- Lofting
- Fumigation
What is Looping?
Strong instability. Occurs on fine summer days with large free convection (surface being heated). The loop is created by turbulent eddies that are larger than the plume diameter, thus the plume is carried up and down, and gets larger as it travels
How are looping plumes destroyed?
Destroyed at faster rate with increasing surface roughness and increasing wind speed
What is Coning?
Cloudy and windy conditions with stability close to neutral. Spreads in all directions equally due to turbulence and molecular diffusion
How are Coning plumes destroyed?
Since the cone gets larger downwind, the plume may hit the ground where there is no forced convection
What is Fanning?
Strongly stable atmosphere (eg. With an inversion). Turbulence is weak, thus little motion acting on the plume. Stable air keeps the plume from mixing and as a result the plume thins. May form v-shape when viewed from above
What is Lofting?
Occurs in the early evening when a nocturnal radiation inversion is building up from the surface. Moves pollutants upwards and is the “best” case scenario for ground level
What is Fumigation?
The opposite of lofting. Occurs when an inversion ‘lid’ obstructs upward dispersion of the plume, but unstable conditions allow for mixing below.
How does the temperature structure for fumigation arise in rural areas?
The nocturnal inversion layer gets eroded away by daytime warming of the surface
How does the temperature structure required for fumigation arise in urban areas?
Fumigation can occur due to the influence of the interaction of buildings (cavity flow)
What is the Standard Odour Unit (SOU)?
The amount on odorous substances per meter cubed of air that would create a minimum detectable odour to an average person
What are the major sources of SOU?
Sewage treatment facilities, meat processing, agricultural activities
How can SOU’s be reduced?
Thought the use of biological or chemical filters. Chemical masking can also be done
How are winds in the urban environment categorized?
According to its position relative to an obstruction such as a building
What is undisturbed flow?
Upwind air flow that is not influences by the obstruction
What is displacement flow?
Air flow moving above, down the front of, or around the side of an obstruction
What is cavity flow?
Air flow that moves as turbulent eddies in the leeward side of an obstruction
What is wake flow?
Air flow that was once displaced, but is now returning or has returned to undisturbed flow
Where does displacement flow against a building generates relatively high pressure?
Over much of the surface of the windward wall, highest pressure is typically near the upper middle portion of the wall (stagnation point)
What affect does the stagnation point have on wind velocities?
Air pressure decreases and wind velocities increase away from the stagnation point
What is air flow in urban areas in dependent on?
The geometry and spacing of the buildings
What is isolated roughness flow?
Refers to flow in urban areas that follows a pattern similar to a series of isolated buildings. H/W= 0.4 (cubic buildings) and 0.65 (row)
What is wake interference flow?
When the wake flow of one building interferes with the flow regime of the building down wind. H/W= 0.7 (cube) and 0.65 (row)
What is skimming flow?
Main air flow begins to skim over buildings.
H/W >0.7 (cubic) and >0.65 (row)
What is lee vortex flow?
Some flow becomes turbulent eddy flow between buildings (usually streets), it is enhances and partially causes by the presence of the down-wind buildings
How do tall buildings affect wind flow?
Since the stagnation point of buildings is about 3/4 H, much of the deflected flow moves in a downward motion enhancing the vortex flow
How does air pollution and building spacing+geometry affect skimming flow?
Pollutants become trapped between buildings
How does air pollution and building spacing+geometry affect isolated roughness flow and wake interference flow?
Increases dispersal of pollutants; however contamination still great a leeward sides of buildings
How do tall buildings influence air pollution?
When it emits pollutants, the cavity flow or lee vortex flow may give rise to a ‘down wash’ of pollutants. In order to avoid this the stacks should be high enough so that the emissions feed into the displacement flow
How is air circulation affected by urban density?
Air flow (both speed and direction) is highly variable due to differential size, orientation, and shape of buildings and streets
Particulate species include…
Dust and total suspended particulates
What is dust?
(>20 micro meters) Principle source is unsaved roads and bare soul. It is more of a irritant than a health problem
What are total suspended particles?
(<20 micro meters) produced from bare soil surfaces, industrial and manufacturing activity as well as the burning of fissile files
What are the total suspended particles less than 10 micro meters in size?
Referred to as PM10, these particles can cause respiratory problems in humans
What are the two majors processes that form secondary pollutants?
1) reaction of SO2,NOx and TSP and high atmospheric moisture to form concentrations of SO4-2 and NO3- as particulate matter and liquid droplets. This is known as smog (London type smog)
2) photochemical smog forms as a combination of NOx, VOC and sunlight through a series of complex chemical reactions (LA type smog)
What is VOC?
Volatile Organic Compounds which react to sunlight to produce secondary pollutants
What is the most common secondary pollutants in the urban atmosphere?
OZone (O3). It’s formation involves some combination of sunlight and NO, NO2 and O2 and or VOC’s as well as a catalyst such as carbon dioxide
How is NO2 formed?
From traffic in early morning rush hours becomes NO2 with the addiction of O. NO2 concentrations peak about 2 hours after rush hour
What is indoor air quality linked too?
Outdoor air quality, as ventilation brings outdoor air and pollutants inside
In addition to ventilation, indoor air quality is a function of…?
1) Number and strength of sources within the home
2) Level of air filtration
3) Maintenance of home
4 Hygiene and cleaning habits
5) Seasons (winter= higher levels of pollutants due to heating)
What are the 6 sources of indoor pollutants?
1) Outdoor air
2) Cigarette smoke
3) Gas stoves, gas heaters, and Kerosene heaters
4) Ceilings and walls
5) Carpet and pets
6) Floors
What is evaporation?
Urban surfaces tens to be warmer than rural surfaces and thus provide more energy for evaporation. This evaporation rates are higher after a storm in an urban area
What is Interception?
Fewer trees result in more water reaching the urban surface than a rural surface. However, buildings may intercept a great deal of precipitation
How are transpiration and soil water affected in dense urban centres?
Soil water usually replenished through stem flow from a few trees or directly under drainage gutter outlets. Transpiration reduced because there are fewer trees
How are transpiration and soil water affected in subarbs?
Although fewer trees than naturally forested areas, transpiration and soil moisture may be greater from these areas
What is field capacity?
The maximum volumetric water context of a soil once all gravitational water has drained. Expressed as a percentage.
What is gravitational water?
Excess water. Large quantities of gravitation water results in leaching of the soil nutrients
What is groundwater?
Source of water for many residential and industrial processes.
What is runoff?
Increase in urban areas due to:
Increased precipitation, decreases evapotranspiration
Impervious surfaces
Output of water pipes into city
As volume increases so does its speed
What are the two types of pollutants concentrations?
1) Gaseous concentrations
2) Concentrations of portide matter in the atmosphere
What are gaseous concentrations?
Usually described in parts per million (ppm) or some variant
What are concentrations of portide matter in the atmosphere?
Usually described as a weight per unit volume (meters cubed of air)
What are primary pollutants?
Those that are emitted directly into the atmosphere from surface sources
What are secondary pollutants?
Created through chemical reactions between primary pollutants and atmospheric components such as water vapour
What are the 4 main groups (gaseous) of benchmark primary pollutants?
Carbon species, sulfur species, nitrogen species, particulate species
What are carbon species?
Include carbon monoxide and non-methane hydrocarbons. Major sources are traffic emissions, oil refiners, and oil processing
What are sulfur species?
Principal pollutant is sulfur dioxide, produces during the burning of fissile fuels. Major sources are industrial stacks
What are nitrogen species?
Major pollutants are nitric oxide and nitrogen dioxide.
What are particulate species?
Made up of dust and total suspended particulates
